1. Field of the Invention
The present invention relates to a focus detecting apparatus. More particularly, the present invention is directed to improvements in a type of focus detecting apparatus in which the detection of in-focus position is carried out by detecting defocused object images at two positions of substantially equal distance from the focal plane of an image forming optical system before and behind the focal plane.
2. Description of the Prior Art
Focus detection apparatus of the type mentioned above is known in the art and the principle of such a focus detection apparatus is shown in FIG. 1 of the accompanying drawings.
In FIG. 1, L designates an objective lens and F is its predetermined focal plane. P.sub.1 before the focal plane F and P.sub.2 behind the focal plane F are two positions substantially, equally distant from the focal plane F. The states of object images formed at the two positions P.sub.1 and P.sub.2 are detected and compared with each other thereby detecting the in-focus position. This focus detecting apparatus assumes that the two defocused object images are symmetric relative to the point in focus. Thus, strictly speaking, the defocused image before the in-focus position and that behind it must be entirely the same with respect to each other so that the amount of defocus from the in-focus position is the same for the two defocused images.
Lenses without any aberrations can satisfy the above requirement. However, objective lenses generally have some aberrations and therefore the two defocused object images before and behind the focus point can not be symmetric. The two images have different shapes even when they are in positions where their defocus values are equal. This will be described in detail with reference to FIG. 2.
FIG. 2 shows the convergence state of parallel rays passed through the lens L at height h from the optical axis 0 of the lens L. If the lens L is a lens without any aberration, then all the rays passed through it at the respective heights of h would be focused at one point on the optical axis. However, as shown in FIG. 2, the lens L like a common lens has spherical aberration. Therefore, the rays of light can not be focused at one point. For this reason, the cross sections A, B, C and D, E, F of the beam spot taken along planes normal to the optical axis 0 are not symmetrical in shape relative to the focal plane. The two defocused object images before and behind the focal point are not exactly in the same state with respect to each other even when they are in positions of equal amount of defocus from the focal point. This asymmetrical relation between the states of two defocused images causes a detection error in focus detection.
In addition, it is known that if the spatial frequency of an object is higher than a certain level, then there is produced a phenomenon of "pseudo-resolution". If positions at which such pseudo-resolution is produced are asymmetric relative to the focal plane, then the image formed at one of the positions is of pseudo-resolution whereas that at the other position is an image of normal resolution. In this case, the focus detecting apparatus no longer holds the aforementioned preposition that defocused images before and behind the focal plane should be symmetrical relative to the focal plane. This constitutes also a major cause for wrong detection.